Sphingosine 1-phosphate (S1P) is a bioactive lipid mediator found in high levels in the blood. Signaling by S1P through its G protein-coupled receptor S1P1 is critical for the emigration of T cells from lymphoid organs into circulation. FTY720, a drug that targets the S1P/S1P1 pathway, induces immune suppression in transplantation and autoimmune diseases. In the peripheral lymphoid system, S1P1 is expressed on naive T cells but lost upon T cell activation. To determine the significance of S1P1 downregulation and function of S1P1 in peripheral T cells, we developed transgenic mice (S1P1-Tg) that constitutively express S1P1 in T cells. S1P1-Tg mice had impaired clearance of Leishmania major infection and defective contact hypersensitivity reaction, which were associated with reduced numbers of antigen-activated T cells in the draining LN. These results demonstrate that S1P1 signaling regulates systemic trafficking of peripheral T cells and primary immune responses, and highlight that control of levels of S1P1 expression represents an important mechanism of immune regulation. Interestingly, S1P1-Tg mice developed a late-onset systemic autoimmune disease. Accordingly, T cells from S1P1-Tg mice were hyper-responsive to TCR stimulation and resistant to tolerance induction. From these findings, we formulate our central hypothesis that S1P1 mediated cellular and molecular pathways in T cells are critical regulators of systemic autoimmunity and lupus pathogenesis. We further hypothesize that S1P1 signaling abrogates peripheral tolerance mechanisms by affecting cellular pathways in T cells that contribute to systemic autoimmunity, and that S1P1 regulates intrinsic T cell functions through signaling pathways and transcriptional mechanisms. To test these hypotheses, we will: Specific Aim 1: Establish the functional significance of S1P1 signaling in peripheral tolerance escape and the pathogenesis of murine lupus in vivo. Specific Aim 2: Identify cellular mechanisms mediated by S1P1 signaling in autoimmune regulation. Specific Aim 3: Identify S1P1-mediated downstream signaling pathways and gene targets. The long-term goals of our studies are to elucidate pathways regulated by S1P signaling in the immune system. Our studies promise to provide new insights into mechanisms and therapies of autoimmune disease. [unreadable] [unreadable] [unreadable] [unreadable]